(1) Field of the Invention
The present invention relates to a cordless telephone composed of a handset and a base unit which functions as a cradle for holding the handset for charging.
(2) Description of Related Art
In some countries, for example, in North American countries, base units and handsets in cordless telephone systems can be purchased separately, and base units and handsets of same models can establish cordless links (also referred to as parent-child relationships, where a parent represents a base unit and a child represents a handset) so that the persons at the handsets can receive incoming calls and make phone calls through the cordless links. A base unit and a handset in a parent-child relationship preserve the same security code (also referred to as security ID) which is used for identifying the parent-child relationship.
The following is a description of the first and second prior arts which are based on the above technique where a base unit and a handset in a parent-child relationship preserve the same security ID.
The first prior art is Japanese Patent Publication No. 5-41051 that discloses a cordless telephone. The cordless telephone is composed of: a base unit which is connected to a telephone line and is used as a cradle for holding a handset for charging; and the handset that operates on a rechargeable battery.
The base unit generates a new security ID using a random number and stores the generated security ID into a memory and transmits the generated security ID to the handset each time the base unit starts charging the handset. The handset receives the new security ID from the base unit and stores the received security ID into a memory after the charge begins.
As described above, the cordless telephone disclosed in the first prior art, in which both the base unit and the handset store a new random number as the security ID each time the handset is placed on the base unit and starts being charged, has the following merits.
When another handset, which has not been in the parent-child relationship with the base unit, is placed on the base unit, the other handset and the base unit establish a parent-child relationship. When this happens, the user of the original handset recognizes an irregularity since the original handset becomes inoperable. The user then can resume the parent-child relationship by placing the original handset on the base unit, excluding the other handset. This means that, for example, when another handset is put on the base unit by an ill-intentioned person to use the telephone, making a charge to the user""s account, the user can notice the irregularity and resume the parent-child relationship by placing the original handset on the base unit, excluding the other handset.
The second prior art also discloses a cordless telephone which is composed of: a base unit which is connected to a telephone line and is used as a cradle for holding a handset for charging; and the handset that operates on a rechargeable battery. Both the base unit and the handset have a wired interface through which serial data is transferred between them. The security ID is transferred between them while the handset is placed on the base unit. The handset includes a rewritable nonvolatile memory such as EEPROM (Electrically Erasable and Programable ROM).
The base unit includes a counter for counting the number of times the base unit has transmitted the security ID to the handset. When the number counted by the counter has not reached a predetermined number (e.g., xe2x80x9c10xe2x80x9d) when the base unit starts charging the handset, the base unit sends the security ID currently stored in the memory to the handset; and when the number counted by the counter has reached the predetermined number, the base unit generates a new security ID using a random number and sends the newly generated security ID to the handset. Upon receipt of a security ID from the base unit, the handset stores the received security ID into the nonvolatile memory (EEPROM) only when the received security ID is different from a security ID which is stored in the nonvolatile memory.
The first prior art, however, has a problem that when a flash memory is used as the memory for storing the security ID, the flash memory loses its life each time the security ID is updated for each charging since the flash memory can be rewritten only a limited number of times.
According to the second prior art, when another handset is put on the base unit and establishes a parent-child relationship with the base unit based on an illegal intention, both the original handset and the other handset can be used until the number in the counter reaches the predetermined number, and during this period, the user cannot detect the irregularity and fails to exclude the other handset.
It is therefore at object of the present invention to provide a base unit and a handset for use in a cordless telephone which allows the user of the handset to recognize that another handset has established a new parent-child relationship with the base unit, without needless loss of life of the memory.
The above object is fulfilled by a base unit included in a cordless telephone which also includes a handset cordlessly linked to the base unit, the base unit functioning as a cradle for holding the handset for charging, the base unit comprising: a storage means for storing a security ID; a detection means for detecting a start of a charge which occurs when a handset is placed on the base unit; a first transmission means for transmitting the security ID stored in the storage means to the handset when the detection means detects the start of the charge; a generating means for generating, when a response from the handset is not received within a predetermined time, a new security ID which is different from the security ID stored in the storage means; a second transmission means for transmitting the new security ID to the handset; a judging means for judging whether a response corresponding to either the security ID transmitted by the first transmission means or the new security ID transmitted by the second transmission means has been received from the handset; a link means for establishing a cordless link with the handset when the judging means judges that the response has been received from the handset; and an update means for updating the security ID stored in the storage means to the new security ID when the judging means judges that the response has been received and when the received response corresponds to the new security ID.
With the above construction, the base unit transmits the security ID to the handset after a charge starts, uses the security ID without updating when the handset returns a response in a predetermined time period (when the transmitted security ID matches the security ID stored in the handset), and transmits a new security ID to the handset when the handset returns no response in the predetermined time period (when the transmitted security ID does not match the security ID stored in the handset).
With the above arrangement, the security ID is updated to a new security ID only when the transmitted security ID does not match the security ID stored in the handset, that is, when another handset is placed on the base unit; and the security ID is not updated when the two security IDs match. This prevents the memory from being rewritten and losing life needlessly.
In the present construction, the security ID is updated to a new security ID whenever the transmitted security ID does not match the security ID stored in the handset. This enables the user of the original, legitimate handset to recognize the presence of another, illegitimate handset without difficulty since the original handset becomes inoperable.
The above object is also fulfilled by a handset included in a cordless telephone which also includes a base unit which is cordlessly linked to the handset and functions as a cradle for holding the handset for charging, the handset comprising: a storage means for storing a security ID used for identifying a parent-child relationship between the base unit and the handset; a detection means for detecting a start of a charge which occurs when the handset is placed on a base unit; a first reception means for receiving a first security ID which is transmitted from the base unit after the detection means detects the start of the charge; a second reception means for receiving a second security ID transmitted from the base unit; a judging means for judging whether the first security ID matches the security ID stored in the storage means; an update means for updating the security ID stored in the storage means to the second security ID when the judging means judges that the first security ID does not match the security ID stored in the storage means; and a link means for requesting the base unit to establish a cordless link between the base unit and the handset in response to the first security ID when the judging means judges that the first security ID matches the security ID stored in the storage means, and in response to the second security ID when the judging means judges that the first security ID does not match the security ID stored in the storage means.
The above construction provides the same effect as the base unit.